Air pollution in the U.S. is better than it was a decade ago, but a staggering 147.6 million Americans—47 percent of the country—live in places where air quality is often too dangerous to breathe, according to the American Lung Association's State of the Air report.

The new study looked at a three-year span of 2010, 2011 and 2012, and compared those figures to the 2013 survey, which looked at 2009-2011. Even though the larger trend since the 1970s is towards much-improved air for everyone, the most polluted cities actually saw a slight decrease in air quality compared to the 2009-2011 period.

Why is our air getting (relatively) worse? Carbon emissions and vehicle emission regulations are still lagging in many parts of the country. A surprisingly old-school form of pollution, wood-burning fires, have devastated air quality in some cities due to population growth. But we can also blame our changing climate: High ozone, especially in warm cities, is likely due to the exceptionally high temperatures during 2012 since sunlight and heat exacerbate ozone levels.

While our beloved sister site had one take on the data, the question of which U.S. cities are the most polluted is actually a bit more nuanced. The report looks at three different indicators for overall air quality: ozone levels, year-round particle pollution, and short-term particle pollution. L.A. ranked at the top of ozone pollution, while the Fresno-Madera region of Central California ranked at the top of both year-round and short-term pollution. So while L.A. is not categorically the worst it's definitely in the top five, depending on how you look at it. The argument could also be made that since L.A.'s metropolitan area is over 18 million people, which is many times larger than any of the other top offenders, L.A.'s bad air is putting far more people at risk.

Is this study helping you breathe easier? I've posted the rankings below, and you can head over to the website to view the cleanest cities, or read the report for full details. [State of the Air]